Flip chip technology has been widely used as it allows a high I/O count, high density interconnection scheme with proven performance and reliability. Solder bumps are deposited on contact pads on both chip surfaces and substrate surfaces, and then the chips are flipped and positioned such that the solder bumps are aligned with matching pads of an external circuit. Solder reflow completes the interconnection process, after which underfill material is introduced to fill the spaces about the interconnections.
In ball grid array (BGA) technology, metal balls comprised of solder are employed for packaging integrated circuits on a printed circuit board. A chip is attached to one side of a substrate. Solder balls are secured to the opposite side of the substrate. The substrate contains traces that electrically connect the chip to the ball grid. The package comprising the chip, substrate, and ball grid array is positioned on a printed circuit board that has contact pads that match the ball grid array. Once properly positioned, the assembly is subjected to heat such that the solder melts. The assembly is then cooled so that the solder cools and solidifies, thereby connecting the package to the circuit board. An assembled board is known as a circuit card assembly (CCA).
Semiconductor chips can be assembled with cored or coreless substrates during packaging. A cored substrate typically contains a layer of material that provides mechanical strength to the substrate. An epoxy-glass reinforced layer is used in some substrates to help prevent bending or warping during bonding or dicing. Coreless substrates, which are also referred to as coreless laminates, lack such a core layer and are more subject to warping than cored laminates. Clamping fixtures have been employed to hold coreless substrates during assembly. Coreless substrates can offer lower cost, thinner package thickness for mobile applications, and some performance advantages over cored substrates.
FIG. 5 is a flow chart showing a conventional process flow employed in fabricating a circuit card assembly. A laminate panel is obtained followed by pre-solder bumping on the front side (the chip attachment side) of the laminate panel. Singulation of the laminate panel is performed by known techniques such as scribing/breaking, sawing, and/or laser radiation. Flip chip module assembly includes joining the chip to the singulated laminate and applying underfill. The assembly modules are mounted to a fixture and BGA bumping on the back side of each module is performed. Second level packaging as described above is employed to produce the circuit card assembly.
Ball grid array solder bumping is employed for second level packaging of module assemblies on a card to fabricate a circuit card assembly. Presently BGA bumping is performed by solder ball attachment following first level packaging. Five steps are generally required for BGA bumping: 1) assembling an array of single modules on a fixture; 2) dispensing tacky flux on the BGA pads using a first mask; 3) placing solder balls using a second mask; 4) solder reflow; 5) cleaning flux residues. FIG. 6 is a flow chart showing steps 2-5, which corresponds to the BGA bumping step shown in FIG. 5. Card assembly follows these steps. The steps are time consuming and impair process flow. The solder balls can move during the reflow process, causing unwanted bump bridging. Moreover, as the pitch of BGA pads becomes finer, the ball size decreases and placement of the solder balls becomes more difficult. Ball sizes of 300 μm and pitch less than 500 μm are exemplary. Placement of solder balls in assemblies using coreless laminates is also problematic due to warpage of such laminates, particularly with solder balls having diameters below 500 μm.
Land grid array (LGA) assemblies are also employed for second level packaging of chip modules to printed circuit boards. Such assemblies may include an interposer or socket having an array of embedded and electrically conductive contacts that are exposed on the front and back faces of the interposer or socket housing. The exposed contacts mirror those of the LGA module and the LGA contact pads on an opposing surface of the printed circuit board. Alignment of the electrical contacts is required when mounting the LGA module.